This invention relates to a magnetic bearing assembly which has at least two actively regulated bearings for stabilizing a rotor in directions that are substantially perpendicular to one another and to the axis of rotation. The bearings are arranged substantially in a radial bearing plane.
A known bearing assembly of the above-outlined type is disclosed, for example, in German Laid-Open Application (Offenlegungsschrift) No. 2,649,182. The actively regulated magnetic bearings are provided with control coils which are energized as a fucntion of signals transmitted by sensors which, in turn, respond to the radial position of the rotor. The control coils and, in some structures, one or more permanent magnets generate a magnetic flux in the disc-shaped or ring-shaped pole pieces of the stator as well as in a particularly U-shaped rotor ring. Thus, in the annular air gaps between the pole faces of the pole pieces and the rotor ring a substantially radially oriented magnetic field prevails. Dependent upon the energization of the control coils, radially oriented forces can be exerted on the rotor ring and the entire rotor.
A magnetic bearing assembly of the above-outlined type is also generally designated as a bi-axially active bearing assembly. Axial displacements of the rotor as well as tilting motions of the rotor about the radial axes are, in contradistinction, stabilized in a passive manner. This means that non-controlled resetting forces or, as the case may be, resetting torques become effective which, similarly to forces derived from resetting springs, return the rotor from its deviated position into a desired position predetermined by the geometry of the pole pieces and the pole ring. Based on the mass of the rotor there is thus provided a system which is capable of oscillation and which is substantially undampened.
Further, a rotor supported in the above-outlined manner has characteristic natural frequencies about the radial axes, namely a precession frequency and a nutation frequency. Even at low rpm's of the rotor precessional oscillations may lead to an instability of the bearing system; the critical rpm's are, however, traversed sufficiently fast in most cases. With increasing rpm's, however, the nutational oscillations, with their increasing amplitudes, pose serious difficulties; there are thus additional measures required to ensure that the critical rpm's are traversed or that the rpm can be further augmented. Furthermore a device including a rotating member supported by two magnetic bearings is shown in the Habermann et al U.S. Pat. No. 3,787,100, whereby the magnetic bearings are disposed in an axial distance along the axis of rotation of said rotating member. Each bearing has sensing means providing signals representative of the relative position of the member with respect to said common axis of the two bearings along first and second direction perpendicular to the axis and to each other. Said signals are fed to a regulating device which is formed as a feed-back loop controls biasing means for controlling the force generating means such as control coils. The regulating device comprises several summation and subtracting circuits as well as phase advance networks having an appropriate response so that the rotating member can be centered in its normal position and precession and nutation movements can be dampered. In consequence of the two bearings disposed in an axial distance to another there is needed a great axial extension of the rotating member and a considerable constructive expense.